Since ten years, with the progress of the adjuvant chemotherapy we treated conservatively 170 osteosarcomas. We try to review the functional and mechanical results of the knee prosthesis with 5 years of follow-up or more. There were 64 resections, 51 of the lower femur and 13 of the upper tibia. The early postoperative complications are few (less than 2%). The functional results are satisfying with 90% of excellent and good results. We observed 3 local recurrences (2 femur, 1 tibia). The survey is really increased by the adjuvant chemotherapy with a rate of 70% at 5 years. However for us the problem is the evolution of our reconstitution prosthesis. 3 kinds of mechanical failures occurred: a wear of the hinge, a fracture of the stem and a loosening of the massive component. The Guepar hinge was finally a good solution when it was metallic, but all the hinges with metal plastic. Articulation needed reinterventions because of quick wearing. For the tibial reconstitution prosthesis we did not observed any problem but for the femoral massive prosthesis, the failures were more frequent, perhaps because the mechanical situation is different. We observed 4 fractures of the stem, 5 loosening of femoral stem (bipolar loosening in one case). In all except one, the reintervention gave a good result. Finally, the result seems to be encouraging, the progress of the implant (Titanium stem, modular prosthesis, semi-constrained knee) may again decrease the rate of mechanical failures.